This invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with:
input terminals for the connection of a supply source, PA1 lamp-connection terminals for the connection of the lamp, PA1 switching means for operating the lamp at a nominal lamp voltage Vla during stable lamp operation, and for generating an open circuit voltage Vo on the lamp-connection terminals before the lamp is ignited, and PA1 a control circuit for controlling the switching means, and provided with means BM for limiting the voltage on the lamp-connection terminals. PA1 extinguished state wherein no current flows through the lamp; the voltage across the lamp is equal to the voltage which is generated as the open voltage Vo by the circuit arrangement; as a result of an applied voltage pulse, breakdown takes place; a glow discharge is formed in the lamp which is accompanied by a very small current through the lamp and a relatively high voltage across the lamp; PA1 transition to an arc discharge; this transition is accompanied by an abrupt decrease of the voltage across the lamp and a corresponding current peak through the lamp. Subsequently, as a result of a gradual increase of the voltage across the lamp to the nominal lamp voltage Vla, the stable operating condition of the lamp occurs.
A circuit arrangement of the type mentioned in the opening paragraph is known from WO 97/39605. The known circuit arrangement can suitably be used to operate and ignite, inter alia, high-pressure sodium lamps and metal-halide lamps. The means BM of the known circuit arrangement provide for a limitation of the voltage on the lamp-connection terminals after a predetermined time period to a value Vb, which is such that Vla&lt;Vb&lt;V0. In general, said lamps are provided with a discharge vessel wherein, during operation, an electric discharge is maintained, said discharge vessel being enclosed, with intervening space, by an outer bulb. Particularly metal-halide lamps have very good color properties and a high specific light output, so that said lamps can very suitably be used, inter alia, for interior lighting. When such a lamp reaches the end of its operational life, frequently leakage of the discharge vessel occurs. As a result of the generally very small dimensions of the lamp, said leakage may readily result in the formation of a discharge in the outer bulb, in particular during igniting the lamp. Such a discharge in the outer bulb may lead to such a substantial temperature increase of the lamp cap that there is a risk that the lighting unit wherein the lamp is mounted starts burning. Although, in the case of the known circuit arrangement, the means BM will reduce the voltage on the lamp-connection terminals to the value Vb after the predetermined time has elapsed, so that in general any discharge formed in the outer bulb will be extinguished, but this proves insufficient to ward off the risk that the lighting unit starts burning. This is a drawback. cl SUMMARY OF THE INVENTION
It is an object of the invention to obviate the above-described drawback.
In accordance with the invention, this object is achieved in that a circuit arrangement of the type mentioned in the opening paragraph is characterized, as a circuit arrangement in accordance with the invention, in that the circuit arrangement is provided with means M for detection of a transition from a glow discharge to an arc discharge. A transition from a glow discharge to an arc discharge is always accompanied by a current peak. On the one hand, such a current peak leads to a substantial dissipation, on the other hand, the occurrence of a current peak is a phenomenon which can be detected relatively easily. When a discharge lamp is ignited, the following phases can be distinguished:
In lamps used in practice, the voltage level across the lamp during glow discharge is of the order of 200 V or more. The transition to the arc discharge is accompanied by an abrupt decrease to a level ranging from 20 V to 100 V, so that a current peak occurs. When a discharge takes place in the outer bulb of the lamp, a similar succession of events takes place. In this case, the transition from glow discharge to arc discharge is accompanied by a current peak of the order of 20 to 25 A. In general, the discharge in the outer bulb is extinguished after the current peak, whereafter a repetition of events takes place.
In an advantageous embodiment, the means M generate a signal S upon detection of the transition. Preferably, the means BM can suitably be used to limit the voltage on the lamp-connection terminals in dependence upon the signal S. In a very suitable embodiment, the means BM comprise a counting register for counting and registering the detected transitions. For this purpose, the means BM preferably comprise a timer. In a favorable embodiment wherein limitation of the voltage at the lamp-connection terminals takes place after a time period, which is set and measured by the timer, has elapsed, the signal S provides for accelerated counting by the timer. As a result, the occurrence of an arc discharge in the outer bulb of the lamp will lead to an accelerated reduction of the voltage at the lamp-connection terminals. It has been found that limitation of the voltage at the lamp-connection terminals to a value Vb such that no discharge can take place in the lamp, is sufficient to also preclude breakdown in the outer bulb. In another embodiment, switching-off of the circuit arrangement takes place.